Discharge lamps in general have a discharge volume enclosed by a discharge vessel, which is filled with a discharge gas comprising typically inert gases and additives necessary to sustain discharge inside the discharge vessel. The discharge takes place typically between electrodes, which extend into the discharge volume and are generally of tungsten, tungsten alloy with or without a further additive or sheath. The electrodes are held and surrounded by the glass material of the discharge vessel in a seal portion. In order to achieve vacuum tight seal, the electrode is configured as a three part electrode assembly comprising an internal part, the actual electrode, an external part (also called the lead-in wire) for connecting the electrode to an external power supply and a seal foil made of a thin metal foil which is electrically connected to both of the electrode and the lead-in wire.
Discharge lamps find application in all areas of lighting technology from home lighting (Metal Halide Lamps) to automobile headlights (High Intensity Discharge Lamps).
High Intensity Discharge (HID) lamps include mercury vapor lamps, (HPM) sodium lamps (HPS), metal halide lamps (MH) and xenon lamps. Xenon lamps are mainly used in projectors because of their high lumen output. In the automotive industry, it is vital to have lamps with a long lifetime, high efficiency and a quick start. HID lamps suitable for automobile reflectors are obtained as a combination of metal halide and xenon lamps. When starting these reflector lamps, the xenon fill in the lamp provides for a quick start and the metal halide fill in the lamp provides for a high efficiency during operation. In the starting period, high voltage pulse trains are used in order to cause a breakdown in the discharge gas between the electrodes. The current flowing through the lamp causes the cathode to reach temperatures typically of 2500° C. Due to the changes of temperature of the cathode over a wide range and the difference of the thermal expansion of the cathode material (usually tungsten) and the seal material (usually quartz glass) cracks are formed in the seal material. Such cracks may propagate to the outer surface causing a communication channel between the inner volume of the envelope and the outer atmosphere. Frequently, such crack propagation due to the mechanical stresses caused by high temporal and spatial thermal gradients at the contact area of the electrodes and the discharge vessel wall leads to leaking channel formation where the high pressure fill material and additives of the discharge vessel are lost, and finally the lamp fails to operate.
Conventionally, the arc tube of a high intensity discharge (HID) lamp, and especially that of a standard metal halide lamp or an HID lamp intended for automotive applications is made of fused silica (quartz glass). An example for the construction of such an arc tube is given in FIG. 1. In the shown example, the arc tube consists of a center portion, the arc chamber 2, where the electric discharge is taking place during lamp operation. The arc chamber is enclosed by an envelope 1 and sealed in a vacuum tight manner at the end portion(s) of the arc tube, that is by the seals or pinch sections 3, also containing the electrode assembly, which is responsible for leading the electric current through the seal. In order to ensure vacuum tightness, the electrode assembly generally consists of three parts as shown in FIG. 1. The electrode 4 shank is usually made of tungsten and ejects charge carriers (electrons) into the discharge plasma. A very thin (some tens of micrometer at maximum) metal seal foil 6 usually made of molybdenum ensures the vacuum tightness of the seal by its plastic and elastic deformations. A metallic lead-in wire 5 of the electrode assembly connects the arc tube to a power supply and may be made of molybdenum.
The temperature of the glass to metal seal area 3 of high intensity discharge (HID) lamps with arc tubes of high wall load can considerably be higher than that of the standard HID lamp products. Wall load means the ratio of the power consumed by the lamp under steady state operation and the arc chamber outer surface area between the two electrode tips. The elevated pinch temperature can adversely affect lamp life, especially in the case of metal halide lamps. For these lamps, one of the main lifetime limiting factors is the kinetics of the chemical reactions between the metal components in the seal—e.g. the molybdenum current leading seal foil 6—and the metal halide dose constituents from the arc chamber. The higher the temperature of the reacting components is, the most severe the effect of these chemical reactions on lamp life.
In general, the requirements with respect to HID lamps with high in-rush and/or steady-state operating currents are extremely high. This is especially true in the case of HID automotive lamps, where the additional requirements of “instant light” generation and “hot re-start” ability imply heavy lamp currents and power overload during the starting and run-up periods of lamp operation. Consequently during the run-up phase, a large part of the electrode bodies are running at much higher temperatures compared to the steady-state conditions. This results in extremely high electrode temperatures at the electrode-to-arc tube wall interface area (at the electrode foot-point), while the surrounding discharge vessel wall temperature is still relatively low.
The high spatial and temporal temperature gradients in the vessel wall surrounding the hot electrode that is in the sealing sections responsible for vacuum-tight closing of the discharge vessel, lead to high mechanical stress levels. The thermally induced additional mechanical stresses can generate micro crack propagation in the pinch seal sections having the glass layered electrode shank when the lamps are repetitively started and then switched off. This is because the shape and dimensions of the micro cracks generated by the thermal expansion mismatch between the electrode and the surrounding glass is very difficult to control. The final result is leaking channel generation where filling gas and dosing constituents of the discharge chamber are lost, and the lamp becomes inoperative. Such early failures or short-life lamp samples severely affect lamp life performance and reliability. Ultimately road safety is affected in a negative way, and maintenance costs are increased.
In order to prevent the fill material from accessing the molybdenum foil 4 in the seal, a quartz glass layer formation around the electrode shank was proposed by U.S. Pat. No. 5,461,277 issued to Van Gennip et al. According to this patent, a glass layer formed on the electrode eliminates the wide channels around the electrode shank, which can usually be observed in conventional discharge lamps. The glass layer is being formed by cracking of the discharge vessel wall around the electrode due to the thermal expansion coefficient mismatch between the quartz glass and the tungsten electrode shank. The advantage of the glass layer comes from the much smaller width of these micro cracks, compared to the ordinary channels around the electrode shank without the proposed glass layer on it. The suggested glass layer structure is a good solution, however it is very difficult to achieve the suggested ideally symmetrical and regular structure, which is necessary in order to avoid crack propagation to the surface. The suggested precise shape and structure can only be achieved by a very expensive manufacturing process and even though a high amount of waste products will be produced. Even the tiniest irregularity in shape and structure of the suggested glass layer may lead to generation of an unwanted crack structure which would propagate to the surface of the surrounding glass wall.
Further, U.S. Pat. No. 5,905,340 discloses a HID lamp with treated electrode. The electrodes are heat treated with high heat, strong vacuum and over an extended period of time before being assembled together to an electrode assembly. In result of the heat treatment, the electrodes will be partially or completely re-crystallized and the out-gas-able components will be removed in order to provide for a better adhesion between the electrode and the seal wall material and to reduce crack failure of HID lamps. This method and the resulting electrode is much too expensive for mass production and the time-consuming heat treating makes the manufacturing process difficult and ineffective. In addition to this, it is not guaranteed that the crack pattern is constant with a controlled crack form.
Thus, there is a particular need for a HID lamp with an electrode seal structure, which is capable of resisting high thermal and mechanical stress due to repeated starting of the lamps and with improved reliability and longer product life. Although micro cracks in the seal region cannot be avoided, it is desirable to have control over the shape and dimensions of the micro cracks in order to avoid micro crack propagation to the outer surface of the HID lamp wall.